In optical systems such as dense wavelength division multiplexing (DWDM) systems, low frequency modulation in the form of pilot tone modulations can be applied to a channel to carry its wavelength and other identification information, which improves fiber link management and facilitates power monitoring. The low modulations are usually added internally via a data driver by modulating its driving amplitude, or externally via variable optical attenuators (VOAs). Modulations added through data drivers require additional hardware and hence cost. The driver also requires feedback control and calibration, increasing system complexity. Modulations added through external VOAs also increase cost and cause difficulty for achieving higher frequency modulation. There is a need for an efficient pilot tone modulation scheme that overcomes such issues.